Bone-contacting implants preferably have an elastic modulus, i.e., stiffness, which is the same as a recipient bone to ensure proper operation and durability. An implant with insufficient stiffness will be unable to withstand the physiological loads, possibly leading to implant failure. However, excessive stiffness in an implant will lead to stress shielding and could result in bone resorption. Stress shielding leads to bone loss when excessively stiff implants prevent natural loading on the recipient bone. Implants made from relatively stiff polymer materials such as polyetheretherketone (“PEEK”) are commonly used with bone because the elastic modulus of PEEK is similar to cortical bone.
Implants, such as femoral components for partial or total knee replacements, are commonly attached to the recipient bone by long anchor pins and further secured to the bone by bone cement such as polymethylmethacrylate compositions. However, bone cement bonding has been observed to fail in some instances. Porous bone contacting surfaces, fabricated by suitable biocompatible metals such as titanium, have been used on implants to provide improved fixation by allowing bone tissue to grow into the porous structure and secure the connection.
A composite implant made of a PEEK body and a porous titanium core will simultaneously provide sufficient strength and adequate bone fixation. However, the stiffness of titanium is much greater than cortical bone, thus making it difficult to maintain an overall implant stiffness in line with cortical bone. Furthermore, polymeric components, such as those made of PEEK, are commonly formed by injection molding which requires additional rework to parts and molds when injection-molding such components around stiff cores and occasionally even leads to deformation of such cores.